Fix onPageStarted/Finished never called in subclass of CordovaWebViewCl...

...ient

onPageStarted/Finished are methods of XWalkClient which is internal class.
For subclass of CordovaWebViewClient usage, We need to route them out.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1882
To align with the history commit of upstream. Could you please move this line above to "boolean isCurrentlyLoading;".